The position of Steward was already hereditary since the days of his grandfather Pelendur and Mardil inherited the title of Steward from his father Vorondil the Hunter in Third Age 2029. He was Steward during the last part of the reign of King Eärnil II and throughout the reign of Eärnur, Eärnil's son.

The King of Minas Morgul (one of the titles of the Ringwraith) reminded Eärnur of this and challenged him at the start of his reign in 2043. Although Eärnur wanted to fight the King of Morgul he was restrained with difficulty by Mardil. But when the King of Morgul issued another challenge in 2050, Eärnur would not be restrained. He marched to Minas Morgul and was never seen again.

Eärnur had never married and had no children, so Mardil took it upon himself to rule Gondor. There was no one with a clear and unassailable claim to the throne, so the rule of the Stewards was a way of averting civil war. Because the fate of the King was unknown, Mardil made an oath to rule until "the King returns". This oath would be sworn by all of the Ruling Stewards.

Mardil ruled Gondor with a steady hand and earned the epithet Voronwë, which means "the Steadfast" in Quenya.

It was soon learned that Eärnur had met certain death at Minas Morgul, and upon Mardil's death in 2080, his son Eradan succeeded him as the second Ruling Steward of Gondor.